Cancer is an uncontrolled growth of abnormal cells. It is found in animals as well as humans. There are many different types of cancer that are found in animals, symptoms are often similar to those in people (eg. abnormal swelling, unexplained weight loss, lethargy / reluctance to exercise etc).
It is one of the leading causes of death in companion animals such as dogs and cats, it is particularly common in animals that live 10 years or longer. If treatment is appropriate this may include chemotherapy and surgery or radiotherapy.
But I guess you can not answer this statistics question, because people rarely do autopsies on their pets nor do they right down the cause of death.
without these numbers it is hard to know how large are the cancer rates.